1. Field of the Invention
This invention relates to digital video tape recorder apparatus.
2. Description of the Prior Art
To record a television signal using a digital video tape recorder (VTR), an incoming analog television signal is sampled, for example to produce luminance and color difference video samples, these video samples are pulse code modulation coded to produce corresponding binary video samples, and signals representing these binary samples are recorded. On reproduction, the binary samples are usually subjected to error correction and error concealment processes before being decoded to form a replica of the original analog television signal.
If the signals are recorded using a digital VTR having a single recording head, then very high data rates are involved, for example 200 to 400 Megabits/second. Such high data rates cause problems, such as ferrite loss in the recording and reproducing heads, and they also necessitate the use of wide bandwidth amplifiers. It is therefore known to demultiplex the data prior to recording, and to use multiple-head assemblies for recording and reproduction. Thus the data may be demodulated into two or four channels, and the recording and reproducing head assemblies each then comprise two or four heads respectively, so that on recording and reproduction, two or four tracks are recorded on or read from the tape at each pass of the head assembly. The demultiplexed data for recording is supplied to the respective heads in the recording head assembly in some fixed predetermined way.
This will now be described in more detail with reference to FIGS. 1 and 2 of the accompanying drawings.
FIG. 1 shows in the top line a sequence of video samples comprising alternating luminance and color difference video samples, the color difference samples also alternating between C.sub.B and C.sub.R. The figure also indicates a known method of demultiplexing the video samples into two channels for supply to respective recording heads A and B. As shown, the demultiplexing is such that the head A, for example, receives alternating luminance samples and alternating pairs of color difference samples. Alternative methods of demultiplexing are of course possible, and in particular the color difference samples can be differently distributed between the heads A and B. However, for simplicity, attention will now be confined to the luminance samples, although similar considerations apply to the color difference samples.
FIG. 2 shows a spatial array of reproduced luminance samples, each line of crosses in the array representing respective luminance samples along a horizontal scan line of the television picture. Suppose now that one of the two heads in the recording or the reproducing head assembly has failed, for example by becoming clogged, so that no reproduced luminance samples are derived from the corresponding channel. In consequence the ringed samples will be lost from the array. To replace these lost samples using a concealment process, the main possibilities are interpolation in the horizontal direction 3 or interpolation in the positive and/or negative diagonal directions 4 and 5. Despite such concealment, it is difficult to produce a television picture which is significantly better than that which would have been produced if the initial sampling rate had been halved.